In high-speed downlink packet access (HSDPA) release eleven (rel-11), a 4-branch multiple-input and multiple-output (MIMO) transmission scheme is introduced at the Node B (NB) to support transmission of multiple data streams over multiple antennas. Specifically, the 4-branch MIMO transmission scheme multiplexes multiple transport blocks into two data streams, which are transmitted over four spatial layers. The NB will also transmit four common pilots (CPICH1, CPICH2, CPICH3 and CPICH4) over the four transmit antennas to provide 4-branch MIMO channel estimation for 4-branch MIMO capable UEs. UEs configured for higher order MIMO (e.g., higher than 2-branch MIMO) are referred to herein as next-generation UEs. As discussed herein, the terms ‘higher order MIMO’ and ‘multi-branch MIMO’ are used synonymously, and refer to any MIMO technique utilizing more than two transmission branches.
One consideration for the higher order MIMO transmission scheme is the increased inter-channel interference experienced by legacy user equipments (UEs) as a result of transmitting the multiple common pilots. Specifically, UEs lacking higher order MIMO functionality (referred to herein as legacy UEs) may not possess the means (e.g., Walsh codes, sufficient number of receive antennas, etc.) for demodulating the multiple common pilots, and consequently may view the common pilots as interference in the code division multiple access (CDMA) downlink channel. To mitigate the interference experienced by legacy UEs, the multiple common pilots may maintain a low transmit-power level. However, in some instances, the low-transmit power of the multiple common pilot channels will not provide sufficiently accurate higher order MIMO channel estimation for the next-generation UEs. Since increasing the transmit-power of the common pilots is undesirable in so far as it would increase the interference experienced by legacy UEs, other mechanisms for improving higher order MIMO channel estimation for next-generation UEs without significantly increasing the interference experienced by legacy UEs are desired.